In a nuclear power plant, spent fuel taken out of a reactor core after operation of a nuclear rector for a fixed period of time is housed and stored in a spent fuel storage rack placed in a spent fuel storage pool in the power plant until reprocessing of the spent fuel is performed, and thereby the spent fuel is cooled and decay heat removal is performed.
In recent years, housing capacity of the storage pool has been tight, so that space in the storage pool has been utilized effectively to increase storing capacity of the storage pool. Further, there has been a plan to build an interim storage facility outside the power plant as a large-sized spent fuel storage facility to store fuel therein. As a container for transporting the spent fuel to an interim storage facility from the power plant, a cask has been used. It is a lattice-shaped basket that is used when the fuel is housed in the above cask. In the above basket, there is used a neutron shielding material made of an alloy to which boron having neutron shielding capability is added and that is based on austenitic stainless steel being the same as that of the spent fuel rack used in water of the pool in the power plant.
As for the boron-containing austenitic stainless steel, a technique described below has been known. That is, the B-containing austenitic stainless steel is obtained in a manner that 500 μm or less of nitrogen-gas atomized powder containing B, C, Si, Cr, Ni, Mo, N, and O falling within a specific range is filled in a specific mild steel can, and the can is vacuum sealed, and then the nitrogen-gas atomized powder is subjected to a HIP treatment under specific temperature and pressure conditions.
The boron-containing austenitic stainless steel has been used for a nuclear fuel transport container, a spent nuclear fuel storage rack, and the like as a control rod and a shielding material by using neutron absorption capability that boron has.
Conventionally, there has been used a shielding plate based on austenitic stainless steel containing about 1% or more of boron for nuclear reactor control and for neutron shielding. In order to provide the neutron absorption capability to the shielding plate, the more an added amount of boron becomes, the better it is, but a solid solution amount of boron into the austenitic stainless steel is quite small, and most of added boron bonds to chromium to precipitate as boride. Thus, a chromium amount in a parent material is reduced to thereby reduce mechanical strength, ductility, and the like of the shielding plate. Further, as an added amount of boron is increased, this reduction tendency becomes noticeable.
A boron-adding alloy has been originally used for a rack of a fuel housing member of a fuel storage pool provided in a power plant. The rack only needs a certain degree of material property because the rack itself is not aimed at moving, but the rack has been manufactured based on austenitic stainless steel with the emphasis on its service life and safety because of being used in water. However, when the rack is targeted at a spent fuel storage container (cask) for transport, or the like, there is room to consider dropping and impact at the time of transport, and thus it is necessary to further increase a mechanical property of the rack. Further, a dry storage cask based on austenitic stainless steel that is a conventional material has a low thermal conduction property to thus cause a problem that cooling efficiency has to be further improved.